LED Chip-Based Lighting Products And Methods Of Building

ABSTRACT

A method of building a light-emitting diode (LED) based lighting product includes mounting a plurality of unpackaged LED chips or LEDs directly on conductors on a surface of a two-sided panel, integrating the panel with support structure to form the lighting product such that at least one surface of the panel forms an external surface of the lighting product, and coupling a diffuser, with a distance from the diffuser to the surface of the LED chips or LEDs being at least twice an average spacing between adjacent LED chips or LEDs. A method of building a an LED chip-based lighting product includes mounting unpackaged LED chips directly on conductors formed on a surface of a two-sided panel, and mounting a cover plate to the LED chips such that light emitted from the LED chips passes through the cover plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/857,472, filed 16 Aug. 2010. U.S. patent application Ser.No. 12/857,472 claims priority to U.S. Provisional Patent ApplicationSer. No. 61/234,309, filed 16 Aug. 2009 and is a continuation-in-part ofU.S. patent application Ser. No. 12/198,662, filed 26 Aug. 2008. All ofthe above-identified applications and appendices thereto areincorporated herein by reference in their entireties.

BACKGROUND

Light-emitting diode (“LED”) based lighting is increasingly used in bothcommercial and domestic settings due to its efficiency, flexibility andlow toxic material content. Solid-state LEDs are generally manufacturedand packaged like other semiconductor products; that is, the LEDs arefirst fabricated in wafer form, then wafers are diced into individualLED chips that are assembled into individual packages. The packages thenmount into products in a variety of ways. In this way, packaging cost isincurred for each individual LED, with this cost accumulating in eachproduct that uses the LEDs.

Net brightness from a single point source is also an issue with LEDbased lighting. The current trend in solid-state lighting is to employlarge LED chips and/or modules of LED chips that have been incorporatedonto a printed circuit board (PCB) assembly. Management of manufacturingcosts currently favors use of large LED chips (e.g., packaged chips thatconsume about one watt of electrical power and emit about 80 to 300lumens of light) because lower numbers of chips and packages are used ina final product. However, users sometimes find the large LED chipsuncomfortably bright. Furthermore, placement of large LED chips into alight fixture in an arrayed fashion (such as in lines or rectilineargrids) may result in the fixture projecting a distracting distributionof light. Managing heat transfer away from large LED chips and/or thePCB assemblies may also be problematic.

SUMMARY

In an embodiment, a method of building a light-emitting diode (LED)chip-based lighting product includes patterning conductors on an insidesurface of a panel, mounting a plurality of unpackaged LED chipsdirectly on the conductors, and integrating the panel with supportstructure to form the lighting product such that an outside surface ofthe panel forms an exterior surface of the lighting product.

In an embodiment, a light-emitting diode (LED) chip-based lightingproduct includes a panel having an inner surface and an outer surface,the outer surface forming an external surface of the lighting product,conductors patterned on the inner surface, and a plurality of unpackagedLED chips mounted directly to the conductors.

In an embodiment, a light emitting diode (LED)-based lighting productincludes a panel having an inner surface and an outer surface, the outersurface forming an external surface of the lighting product, conductorspatterned on the inner surface, and a plurality of LEDs mounted directlyto the conductors.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an LED-based lighting product, in accord with anembodiment.

FIG. 2 shows a printed circuit board (“PCB”) assembly with LED chipsassembled thereon, in accord with an embodiment.

FIG. 3 illustrates an early stage of fabrication of a PCB, in accordwith an embodiment.

FIG. 4 illustrates components mounted to the PCB of FIG. 3, in accordwith an embodiment.

FIG. 5A is a side view of a cover plate with conductors, in accord withan embodiment.

FIG. 5B is a side view of the cover plate with conductors of FIG. 5A,and shows a phosphor layer formed on a bottom surface of the coverplate, in accord with an embodiment.

FIG. 5C shows a bottom view of the cover plate of FIG. 5A with theconductors formed as two-dimensional shapes on bottom surface thereof,in accord with an embodiment.

FIG. 6A is a side view of a cover plate subassembly that includes thecover plate of FIG. 5A with two sections of a conformal phosphor gelapplied thereto, in accord with an embodiment.

FIG. 6B is a side view of a cover plate subassembly that includes thecover plate of FIG. 5B with two sections of a conformal gel appliedthereto, in accord with an embodiment.

FIG. 6C is a side view of a cover plate subassembly that includes thecover plate of FIG. 5A with two sections of conformal gel and a phosphorlayer applied thereto, in accord with an embodiment.

FIG. 7A shows a cover plate subassembly that includes the cover platewith conductors and phosphor gel of FIG. 6A, with a conductive epoxyapplied in liquid form to the conductors, in accord with an embodiment.FIG. 7B shows a cover plate subassembly that includes the cover platewith conductors, conformal gel and phosphor layer of FIG. 6B, with aconductive epoxy applied in liquid form to the conductors, in accordwith an embodiment.

FIG. 8A shows a PCB assembly formed by mounting the cover plate shown inFIG. 7A, onto the PCB shown in FIG. 4, in accord with an embodiment.FIG. 8B shows a PCB assembly formed by mounting the cover plate shown inFIG. 7B, onto the PCB shown in FIG. 4, in accord with an embodiment.

FIG. 9A shows a portion of a PCB assembly with reflectors, in accordwith an embodiment.

FIG. 9B shows a cover plate subassembly including reflectors, in accordwith an embodiment.

FIG. 9C shows a PCB assembly formed by mounting the cover platesubassembly of FIG. 9B to a PCB with LED chips, and filling the assemblywith fill material, in accord with an embodiment.

FIGS. 10A through 10C illustrate how solder reflow may be utilized toalign LED chips to a PCB, in accord with an embodiment.

FIG. 11A is a plan view of a portion of a PCB with two LED chips and astandoff mounted thereon, for subsequent coupling with cover platesubassembly to form a PCB assembly, in accord with an embodiment.

FIG. 11B is a plan view of a cover plate subassembly ready for couplingwith the PCB of FIG. 11A to form a PCB assembly, in accord with anembodiment.

FIG. 11C shows the cover plate subassembly of FIG. 11B coupled with thePCB of FIG. 11A, to form a PCB assembly, in accord with an embodiment.

FIG. 12A shows a cross-sectional view of a PCB assembly, in accord withan embodiment.

FIG. 12B shows a PCB with LED chips, standoffs and insulating damattached thereto, ready for attachment to a cover plate subassembly toform the PCB assembly of FIG. 12A.

FIG. 12C shows cover plate subassembly with multiple instances ofconductors, phosphor gel and conductive epoxy ready for alignment to,and coupling with, the PCB of FIG. 12B to form the PCB assembly of FIG.12A.

FIG. 13 is a flowchart showing steps of a method of assembling a PCBassembly with LED chips, in accord with an embodiment.

FIG. 14A shows a cross-sectional view of a PCB assembly that includestwo LED chips that each couple to a PCB and to a cover platesubassembly, in accord with an embodiment.

FIG. 14B shows a PCB with conductive epoxy applied in locations facingLED chips and reflectors of a cover plate subassembly, during assemblyof the PCB assembly of FIG. 14A.

FIG. 14C shows a cover plate subassembly with multiple instances ofconductors, LED chips and reflectors attached thereto, ready forattachment to the PCB of FIG. 14B to form the PCB assembly of FIG. 14A.

FIG. 15 is a flowchart showing steps of a method 500 of assembling a PCBassembly with LED chips, in accord with an embodiment.

FIG. 16A illustrates a prior art LED-based lighting system.

FIG. 16B illustrates an LED chip-based lighting product, in accord withan embodiment.

FIG. 17A is a cross-sectional view of a LED chip-based lighting product,in accord with an embodiment.

FIG. 17B is a detail view of a region denoted A within the LEDchip-based lighting product of FIG. 17A.

FIG. 17C is a detail view of a region denoted B within region A of theLED chip-based lighting product of FIG. 17A.

FIG. 18 is a detailed illustration of a portion of an LED chip-basedlighting product, in accord with an embodiment.

FIG. 19 is a flowchart showing steps of a method for building an LEDchip-based lighting product, in accord with an embodiment.

FIG. 20 shows a cross section of an LED-based lighting product, inaccord with an embodiment.

DETAILED DESCRIPTION OF DRAWINGS

The present disclosure may be understood by reference to the followingdetailed description taken in conjunction with the drawings describedbelow. It is noted that, for purposes of illustrative clarity, certainelements in the drawings may not be drawn to scale. Specific instancesof an item may be referred to by use of a numeral in parentheses (e.g.,PCB assemblies 200(1), 200(2)) while numerals without parentheses referto any such item (e.g., PCB assembly 200). Certain drawings label onlyrepresentative instances of an element, for illustrative clarity.

FIG. 1 shows an LED-based lighting product 10. Lighting product 10includes a plurality of LED chips 30 that emit light through apertures25 of a housing 20. Housing 20 may be a metal rail as shown, but mayalternatively be of any desired form or material, and may includetranslucent or transparent materials for LED chips 30 to emit lightthrough, in which case housing 20 need not include apertures 25.

FIG. 2 shows a printed circuit board (“PCB”) assembly 200 with LED chips30 assembled thereon. PCB assembly 200 includes a PCB 40 to which LEDchips 30 mount. PCB assembly 200 is an example of a PCB assembly thatmay be utilized within LED-based lighting product 10.

FIG. 3 illustrates an early stage of fabrication of a PCB 40(1). PCB40(1) includes a substrate 45 and conductors 50. Substrate 45 may be ofknown PCB substrate materials; for example, woven glass impregnated withepoxy (sometimes sold under the trade name “FR4”), cotton paper or matteglass impregnated with epoxy, woven glass impregnated with polyester.Conductors 50 may be metal, and for example may be thick copper tracesthat support thermal transfer in addition to electrical connectivity.Conductors 50 are formed on substrate 45 using known methods of PCBfabrication. Other features may also be formed on substrate 45, forexample fiducial marks may be formed for later use in aligning LED chips30, or cover plate subassemblies (see for example FIG. 6A, FIG. 6B, FIG.9B, FIG. 11B, FIG. 12C and FIG. 14C) with PCB 40(1).

FIG. 4 illustrates components mounted to PCB 40(1), including circuitcomponents 60, LED chips 30 and a standoff 80 that provides electricaland/or mechanical support for a cover plate or cover plate subassembly,as will be further illustrated below. Circuit components 60, LED chips30 and standoff 80 may be soldered, or mounted with conductive epoxy, toconductors 50 (not all conductors 50 are labeled in FIG. 4, for clarityof illustration). Circuit components 60 may, for example, regulate powersupplied to LED chips 30. When conductive epoxy is utilized, the epoxymay be hardened by a thermal bake or by using ultraviolet (“UV”) light.

FIG. 4 and other drawings herein schematically show LED chips 30 ashaving an N region at a “bottom” side of each chip and a P region at a“top” side of each chip, and the assembly sequence shown in FIG. 3through FIG. 8B utilize one “backside” electrical contact and one“frontside” electrical contact. However, the P and N regions may bereversed from the order illustrated, and an LED chip 30 may have both Pand N contacts on a top surface and that both such contacts may couplewith conductors of a cover plate subassembly using the methods discussedbelow (in connection with FIG. 12A through FIG. 12C, for example).Furthermore, as utilized in the present disclosure the “top” side or“frontside” of an LED chip shall mean the side from which light emitsfrom the chip, and the “bottom” side or “backside” shall mean a sideopposite the frontside. That is, the backside is a side from which lightdoes not emit or from which any light that leaks through is not used.

FIG. 5A is a side view of conductors 110 on a bottom surface 103 of acover plate 100 (the terms “bottom surface” and “top surface” areunderstood as being in reference to a final configuration of cover plate100 atop PCB 40, as shown in FIGS. 8A and 8B). Cover plate 100 may beformed of quartz, glass, sapphire, plastic, Mylar, polycarbonate,acrylic, polyester, polyethylene and composites thereof, or othermaterial that is transparent to light generated by LED chips 30. Aspecific material forming cover plate 100 may be chosen to have acoefficient of thermal expansion approximating that of substrate 45, tominimize the possibility of cracking or adhesive failure withenvironmental stresses such as temperature cycling or vibration.Conductors 110 may be formed of metal or may be formed of conductive buttranslucent or transparent materials (e.g., indium tin oxide).Conductors 110 may be formed by conventional methods such as masking andetching such that conductors 110 form a two-dimensional pattern onbottom surface 103 (see FIG. 5C). Besides conducting electrical voltagesand/or currents, conductors 110 may be utilized for identification(e.g., part numbers, barcodes) or for visual recognition and positioning(e.g., fiducial marks for alignment of cover plate 100 to LED chips or aPCB). Cover plate 100 also has a top surface 105 upon which coatings maybe applied, such as for example antireflective coatings to reduce lightreflections at an air interface. Either of bottom surface 103 or topsurface 105 may also be shaped, by methods known in the art such asmolding, embossing, etching, engraving and/or blazing, to form opticssuch as lenses, gratings, Fresnel lenses and the like, to modify lightpassing therethrough by means of refraction or diffraction (see, forexample, FIG. 12A and FIG. 12C).

FIG. 5B is a side view of cover plate 100, similar to FIG. 5A, but withconductors 110 formed on a phosphor layer 122 that covers bottom surface103 of cover plate 100. Phosphor layer 122 may fluoresce whenilluminated by LED chips 30, thus converting some of the light energyemitted by LED chips 30 into longer wavelengths to produce a betterapproximation of white light than the light emitted by LED chips 30themselves. Phosphor layer 122 is shown in FIG. 5B on bottom surface103, but it is understood that alternatively, phosphor layer 122 may beformed on top surface 105 of cover plate 100.

FIG. 5C shows a bottom view of cover plate 100 with conductors 110formed as two-dimensional shapes on bottom surface 103 thereof. Brokenline 5A-5A shows a line of sight along which the views of FIGS. 5A and5B are taken.

FIG. 6A is a side view of a cover plate subassembly 102(1) that includescover plate 100 and conductors 110 with two sections of a conformalphosphor gel 120 applied thereto. One or more phosphors, admixed with agel to form phosphor gel 120, fluoresce under light emitted by LED chips30, like phosphor layer 122, FIG. 5B, discussed above. Phosphor gel 120may also be pliable so that, after assembly, it conforms to surfacecontours of LED chips 30. Portions of phosphor gels 120 are hidden inFIG. 6A behind conductor 110.

FIG. 6B is a side view of a cover plate subassembly 102(1) that includescover plate 100 and conductors 110 with phosphor layer 122 (as in FIG.5B) and two sections of a conformal gel 120′ applied thereto. Utilizingphosphor layer 122 with conformal gel sections 120′ may promotemanufacturing flexibility and reduced cost, since it may not benecessary to remove phosphor layer 122 from areas that do not face LEDchips in a final product, but conformal gel sections 120′ can bepatterned to match an LED layout of a particular product design.

FIG. 6C is a side view of a cover plate subassembly 102(2) having coverplate 100 with two sections of conformal gel 120′ and two sections of aphosphor layer 130. Portions of phosphor layers 130 are hidden in FIG.6C behind conductor 110. It is appreciated that positions of conformalgel 120′ and phosphor layer 130 may be reversed from the positions shownin FIG. 6C, such that conformal gel 120′ is in contact with cover plate100. Phosphor gel 120, conformal gel 120′ and phosphor layer 130 may beformed on cover plate 100 by known methods such as screen-printingand/or photolithography.

FIG. 7A shows cover plate subassembly 102(1) (as shown in FIG. 6A) withconductive epoxy 140 applied in liquid form to locations on conductors110 that correspond to circuit connections of LED chips 30 and standoff80 on PCB 40(1) (as can be seen in FIG. 8A, for example—also see FIGS.11A through 11C, 12A through 12C and 14A through 14C). FIG. 7B showscover plate subassembly 102(2) (as shown in FIG. 6B) with conductiveepoxy 140 applied in liquid form to conductors 110. As in FIG. 7A,conductive epoxy 140 is applied to locations of conductors 110 thatcorrespond to the location of circuit connections of LED chips 30, andstandoff 80, on PCB 40(1). It is understood that alternatively,conductive epoxy 140 may be applied to LED chips 30 and/or standoff 80on PCB 40(1), in locations that correspond to conductors 110 on coverplate 100.

FIG. 8A shows a PCB assembly 200(1) formed by mounting cover platesubassembly 102(1), as shown in FIG. 7A, onto PCB 40(1), as shown inFIG. 4. Cover plate 100 is inverted relative to the position shown inFIG. 7A, aligned to PCB 40(1) such that conductive epoxy 140 contactscircuit connections of LED chips 30 and standoff 80 on PCB 40(1), and isheld in this orientation until conductive epoxy 140 is hardened by usingUV light and/or a thermal bake. FIG. 8B shows a PCB assembly 200(2)formed by mounting cover plate subassembly 102(2), as shown in FIG. 7B,onto PCB 40(1), as shown in FIG. 4. Alignment of cover plate 100 to PCB40(1) and hardening of conductive epoxy 140 are done in similar mannerfor PCB assembly 200(2) as for assembly 200(1). When cover plate 100 andlayers thereon are assembled to PCB 40(1) to form assemblies 200(1) and200(2), gel 120 or 120′ may compress to adjust for a total heightbetween LED chips 30 and cover plate 100 so that LED chips 30 and coverplate 100 are well coupled optically (e.g., through gel 120, gel 120′and/or phosphor layer 130) but only couple mechanically (in a rigidsense) through conductive epoxy 140. That is, gel 120, gel 120′ and/orphosphor layer 130 allow for optical coupling but provide a mechanicaldegree of freedom so that phenomena such as mechanical tolerances,thermal expansion and contraction, and the like, do not exert unwantedforce or pressure on LED chips 30.

It is contemplated that embodiments of cover plate subassemblies 102herein may be utilized for circuitry (e.g., like a PCB) to any extentconsistent with the use of the cover plate itself. For example, circuitcomponents may be attached to conductors 110 in addition to, or insteadof, such components attaching to PCB 40.

Other materials or features may be incorporated into PCB assemblies 200or components thereof, for enhanced reliability and/or performance. Forexample, FIG. 9A shows a portion of a PCB assembly 200(3) withreflectors 150(1). Reflectors 150(1) may be formed of metal (e.g., astamped foil such as aluminum foil), a metal coated plastic (e.g.,metalized Mylar) or micromachined silicon, and may mount with conductors50, as shown, or may mount with cover plate 100, as shown in FIG. 9B.Reflectors 150(1) serve to increase efficiency of PCB assembly 200(3) byreflecting stray light and light emitted from sides of LED chips 30 upthrough cover plate 100. PCB assembly 200(3) also includes a fillmaterial 160 that fills space between a PCB 40(2) and cover plate 100,except space occupied by components such as LED chips 30, standoff 80,phosphor gel 120 and conductive epoxy 140. Fill material 160 may be agel, a fluid, epoxy, a UV curable material such as silicone, or a liquidcrystal material. Fill material 160 advantageously protects and/orpassivates exposed surfaces of LED chips 30 and keeps contaminants outof the space between PCB 40 and cover plate 100. When fill material 160is liquid crystal material, conductors 50 and 110 may be utilized toactivate the liquid crystal material to modulate reflectivity of PCBassembly 200(3). Alternatively, fill material 160 may have a refractiveindex matched to a refractive index of cover plate 100, therebyeliminating a Fresnel reflection that would otherwise occur at aninterface between cover plate 100 and air at bottom surface 103 (e.g.,see FIG. 8A).

FIG. 9B shows a cover plate subassembly 102(3) including reflectors150(2). Like reflectors 150(1) shown in FIG. 9A, reflectors 150(2) maybe formed of metal, a metal coated plastic or micromachined silicon.Reflectors 150(2) attach to conductors 110 of cover plate 100 such thatwhen assembled to a PCB with LED chips mounted thereon, reflectors150(2) are located between the LED chips and increase efficiency byreflecting light through cover plate 100. FIG. 9C shows a portion of aPCB assembly 200(4) formed by mounting cover plate subassembly 102(3)(shown in FIG. 9B) to a PCB 40(3) with LED chips 30, and filling theassembly with fill material 160.

Additionally to the use of reflectors and fill material, LED chips 30may include features and materials that cooperate with the materials andconstruction method detailed above. For example, LED chips 30 mayinclude a phosphor coating and/or index matching gel before mounting toa PCB 40; such coatings may be applied in wafer form for reduced cost.LED chips 30 may also be designed to include features such as fiducialmarks that facilitate alignment of other structures thereto by humans orby machine vision (see for example FIG. 11A). Also, although FIGS. 4,8A, 8B, 9A and 9C herein show LED chips 30 as having an N type bottomlayer accessed through a backside contact and a P type top layeraccessed through a frontside contact, it is appreciated that LED chipsmay include topside contacts for both P and N layers, as illustrated inFIGS. 12A through 12C and FIGS. 14A through 14C.

Other features that may be incorporated into PCB assemblies with LEDchips facilitates alignment among the components thereof. A layout of aPCB assembly 200 may require alignment tolerances among the componentsthereof, leading to the layout being larger when the alignmenttolerances are large. The larger layout may contribute to issues withperformance (e.g., transferring heat away from the LED chips, inabilityto get as many LED chips as desired into a package of a given size)and/or reliability (e.g., larger cover plates and/or PCBs may be moresusceptible to cracking or adhesive failure under stress). Inparticular, features that facilitate self-aligning assembly and/ormachine vision for alignment purposes are now described.

FIGS. 10A through 10C illustrate how solder reflow may be utilized toalign LED chips to a PCB. FIG. 10A shows a portion of a PCB 40(4) thatincludes a substrate 45, and conductors 50(1) sized for coupling of LEDchips 30 through self-aligning solder reflow. Solder may be supplied inthe form of a slug that approximately matches the outline of conductors50(1) where attachment of LED chips 30 is desired. FIG. 10B shows LEDchips 30 and solder 42 placed onto conductors 50(1) with imperfectalignment; note that edges of LED chips 30 and solder 42 do not alignvertically with edges of conductors 50(1). Solder 42 is then heated to amelting point of solder 42, which melts to form a liquid 42′ havingsurface tension, as shown in FIG. 10C. The surface tension is minimizedby reducing area about edges of LED chips 30 and conductors 50(1),pulling LED chips 30 into alignment with conductors 50(1). When liquidsolder 42′ cools and hardens into solid form, LED chips 30 remainaligned and couple with conductors 50(1). The term “solder” herein isnot limited to lead-tin solder but encompasses all equivalent types oflow melting point metals that may include, for example, lead, tin,copper, silver, bismuth, indium, zinc and antimony.

The approach illustrated in FIGS. 10A through 10C is particularlyeffective for smaller LED chips (e.g., LED chips with length and/orwidth less than 250 microns), as an aligning force generated in a givendirection at the edges of liquid solder 42′ is proportional to aperipheral length of each LED chip 30 transverse to that direction,while a mass of each LED chip 30 is proportional to an area of the chip.Therefore, for a square LED chip 30 having sides of length L (as shownin FIG. 10A) a ratio of the aligning force to the mass of a single LEDchip 30 varies as 2 L/L². This ratio is larger for a smaller L, so asmaller LED chip is subject to a higher aligning force in proportion toits mass. It is also appreciated that alternatively, (1) solder 42 maybe plated, or formed by deposition and etching, onto traces 50(1) whereattachment of LED chips 30 is desired, and/or (2) conductors 50 mayextend beyond a desired bonding area for LED chips 30, with a soldermasklayer forming an opening at the desired bonding area.

FIG. 11A is a plan view of a portion of a PCB 40(5) with two LED chips30(1) and standoff 80 mounted thereon, for subsequent coupling withcover plate subassembly 102(4) (see FIG. 11B) to form a PCB assembly.PCB 40(5) includes substrate 45, conductors 50 and a fiducial mark 46,as shown. LED chips 30(1) and standoff 80 couple with conductors 50using solder and/or conductive epoxy. Although conductors 50 are shownas slightly overlapping LED chips 30(1) and standoff 80, this is forillustrative clarity and it is appreciated that conductors 50 may belaid out coincidentally with LED chips 30(1) and/or standoff 80 forpurposes of self-aligning solder reflow, as discussed above inconnection with FIGS. 10A through 10C. Each LED chip 30(1) is shown ashaving a light emitting area 32, a frontside contact 34 and fiducialmarks 36 in dashed outline; it is appreciated that shape, size andposition of light emitting area 32, a frontside contact 34 and fiducialmarks 36 are matters of LED chip layout and may vary from the shapesshown.

FIG. 11B is a plan view of a cover plate subassembly 102(4) ready forcoupling with PCB 40(5) to form a PCB assembly. FIG. 11B shows bottomsurface 103 of cover plate subassembly 102(4) such that subassembly102(4) would be turned over top-to-bottom to couple with PCB 40(5).Cover plate subassembly 102(4) includes cover plate 100 having aconductor 110, phosphor gels 120 and a fiducial mark 146. Conductiveepoxy 140 is shown as being placed on conductor 110 such that epoxy 140will face frontside contacts 34 and standoff 80 when subassembly 102(4)couples with PCB 40(5).

FIG. 11C shows cover plate subassembly 102(4), FIG. 11B, coupled withPCB 40(5), FIG. 11A, to form PCB assembly 200(5). Since cover plate 100is transparent, most elements shown in FIG. 11A and FIG. 11B remainvisible, but conductor 110 is opaque, hiding conductive epoxy 140,frontside contacts 34 and one instance of fiducial mark 36 in FIG. 11C.Alignment of cover plate subassembly 102(4) to PCB 40(5) includesaligning fiducial mark 146 (FIG. 11B) to fiducial mark 46 (FIG. 11A) sothat fiducial mark 46 is also hidden beneath fiducial mark 146 in FIG.11C.

FIG. 12A shows a cross-sectional view of a PCB assembly 200(6). PCBassembly 200(6) has two LED chips 30(2) that each couple to a PCB 40(6)(FIG. 12B) and, using two instances of conductive epoxy per LED chip30(2), to a cover plate subassembly 102(5) (FIG. 12C). PCB assembly200(6) also includes an insulating dam 170 for containing fill material160. Dam 170 is shown in FIG. 12A as attached to PCB 40(6) and to coverplate assembly 102(5) using epoxy 140; in other embodiments dam 170 maybe formed of an electrically conductive material and may attach to a PCBand/or a cover plate using solder. FIG. 12A also illustrates optics 155in the form of a Fresnel lens formed into top surface 105 of cover plate100. FIG. 12B shows PCB 40(6) with LED chips 30(2), standoffs 80 andinsulating dam 170 attached thereto. FIG. 12C shows cover platesubassembly 102(5) with multiple instances of conductors 110, phosphorgel 120 and conductive epoxy 140 ready for alignment to, and couplingwith, PCB 40(6) (FIG. 12B). FIG. 12C shows cover plate subassembly102(5) from the perspective of facing bottom surface 103 of cover plate100; optics 155 are therefore shown in dashed lines where visiblethrough transparent cover plate 100 and phosphor gel 120 in top surface105 (see FIG. 12A). Sight lines 12A-12A in each of FIG. 12B and FIG. 12Cindicate the plane at which the cross-sectional view of FIG. 12A istaken.

FIG. 13 is a flowchart showing steps of one exemplary method 300 ofassembling a PCB assembly with LED chips. Method 300 may, for example,be utilized to assemble PCB assembly 200. Steps 302 through 308 assemblea PCB portion of the PCB assembly, while steps 320 through 326 assemblea cover plate subassembly independently of the PCB portion. Steps 330through 340 join the cover plate portion to the PCB portion to form thecompleted PCB assembly.

Step 302 patterns conductors on a PCB (e.g., patterns conductors 50 onPCB 40) using known methods of PCB fabrication. An optional step 304,shown in a dashed box, attaches circuitry (e.g., circuit components 60)to the PCB, either by soldering or by attaching the circuitry to the PCBusing conductive epoxy. Step 304 is not applicable for products wherecircuit components are not needed or are not implemented on the same PCBas the LED chips (e.g., when PCB 40 includes only LED chips, and circuitfunctionality is implemented elsewhere). Step 306 attaches LED chips(e.g., LED chips 30) to the PCB. An optional step 308 attaches one ormore standoffs (e.g., standoff 80), reflectors (e.g., reflector 150)and/or dams (e.g., dam 170) to the PCB. Steps 306 and 308 may utilizesolder and/or conductive epoxy; when epoxy is used, the correspondingstep may include a thermal bake or UV cure to harden the epoxy.

An optional step 320, shown in a dashed box, forms a phosphor layer(e.g., layer 122) or phosphor sections (e.g., phosphor layers 130) on acover plate (e.g., on cover plate 100). Step 322 patterns conductors onthe cover plate (e.g., patterns conductors 110). An optional step 324attaches circuitry to the cover plate. An optional step 326 forms aconformal index matching gel (e.g., gel 120, 120′) on the cover plate.Step 326 may be omitted (a) for cost savings, (b) when the LED chipsbeing assembled include index matching gel and/or phosphor coatingapplied in wafer form, and/or (c) when fill material is to be utilizedfor purposes similar to those of the index matching gel.

Step 330 applies conductive epoxy to conductors, LED chips, reflectors,dams and/or standoffs on one or both of (a) the PCB prepared as in steps302 through 308, and (b) the cover plate subassembly prepared as insteps 320 through 326. Step 332 flips over the cover plate subassemblysuch that the conductive epoxy applied in step 330 faces thecorresponding locations on the other of the cover plate portion and thePCB. Step 334 aligns the cover plate subassembly with the PCB. Step 336moves the cover plate subassembly and the PCB portion together such thatthe epoxy couples the appropriate locations on the PCB and itscomponents, with the appropriate locations on the cover platesubassembly. It is appreciated that steps 334 and 336 may be iterated,combined, or performed in a different order than that shown in FIG. 9.For example, a crude alignment may be performed first, followed by acrude approach of the cover plate subassembly to the PCB (at which pointthe conductive epoxy may or may not be in contact with both the coverplate subassembly and the PCB), followed by a fine alignment, followedby bringing the cover plate portion and the PCB portion together to afinal distance from one another. Step 338 utilizes UV light or a thermalbake to cure the conductive epoxy applied in step 330, to complete theassembly of the PCB assembly. An optional step 340 applies fill materialto spaces between the PCB and the cover plate, as described inconnection with FIG. 9A.

FIG. 14A shows a cross-sectional view of a PCB assembly 200(7) thatincludes two LED chips 30(2) that each couple to a PCB 40(7) (see FIG.14B) and to a cover plate subassembly 102(6) (see FIG. 14C). PCBassembly 200(7) is manufactured by attaching LED chips 30(2) to coverplate subassembly 102(6) before cover plate assembly 102(6) attaches toPCB assembly 200(7), as described below. PCB assembly 200(7) alsoincludes reflectors 150(3) and 150(4) that each have a height that issufficient for the reflectors to function as standoffs, that is, to seta distance between PCB 40(7) and a cover plate 100 (see FIG. 14C). FIG.14B shows PCB 40(7) with conductors 50 and conductive epoxy 140 appliedin locations facing LED chips 30(2) and reflectors 150(3) and 150(4) asshown in FIG. 14C. FIG. 14C shows cover plate subassembly 102(6) withmultiple instances of conductors 110 fabricated thereon, and with LEDchips 30(2) and reflectors 150(3) and 150(4) attached thereto. Dashedlines within LED chips 30(2) show positions of light emitting areas andfrontside contacts that are on the underside of LED chips 30(2), thatis, facing cover plate 100. Sight lines 14A-14A in each of FIG. 14B andFIG. 14C indicate the plane at which the cross-sectional view of FIG.14A is taken.

FIG. 15 is a flowchart showing steps of one exemplary method 500 ofassembling a PCB assembly with LED chips. Method 500 may, for example,be utilized to assemble PCB assembly 200. Steps 502 through 506 assemblea PCB portion of the PCB assembly, while steps 520 through 528 assemblea cover plate subassembly independently of the PCB portion. Steps 530through 540 join the cover plate portion to the PCB portion to form thecompleted PCB assembly.

Step 502 patterns conductors on a PCB (e.g., patterns conductors 50 onPCB 40) using known methods of PCB fabrication. An optional step 504,shown in a dashed box, attaches circuitry (e.g., circuit components 60)to the PCB, either by soldering or by attaching the circuitry to the PCBusing conductive epoxy.

Step 504 is not applicable for products where circuit components are notneeded or are not implemented on the same PCB as the LED chips (e.g.,when PCB 40 includes only LED chips, and circuit functionality isimplemented elsewhere). Optional step 506 attaches one or more standoffs(e.g., standoff 80), reflectors (e.g., reflector 150) and/or dams (e.g.,dam 170) to the PCB. Step 506 may utilize solder and/or conductiveepoxy; when epoxy is used, the corresponding step may include a thermalbake or UV cure to harden the epoxy.

An optional step 520, shown in a dashed box, forms a phosphor layer(e.g., layer 122) or phosphor sections (e.g., phosphor layers 130) on acover plate (e.g., on cover plate 100). Step 522 patterns conductors onthe cover plate (e.g., patterns conductors 110). An optional step 524forms a conformal index matching gel (e.g., gel 120, 120′) on the coverplate. Step 524 may be omitted (a) for cost savings, (b) when the LEDchips being assembled include index matching gel and/or phosphor coatingapplied in wafer form, and/or (c) when fill material is to be utilizedfor purposes similar to those of the index matching gel. An optionalstep 526 attaches circuitry to the cover plate. Step 528 attaches theLED chips to the cover plate.

Step 530 applies conductive epoxy to one or the other of conductors, LEDchips, reflectors, dams and/or standoffs on the PCB prepared as in steps502 through 508, and/or the cover plate subassembly prepared as in steps520 through 524. Step 532 flips over the cover plate subassembly suchthat the conductive epoxy applied in step 530 faces the correspondinglocations on the other of the cover plate portion and the PCB. Step 534aligns the cover plate subassembly with the PCB. Step 536 moves thecover plate subassembly and the PCB portion together such that the epoxycouples the appropriate locations on the PCB and its components, withthe appropriate locations on the cover plate subassembly. It isappreciated that steps 534 and 536 may be iterated, combined orperformed in a different order than that shown in FIG. 9. For example, acrude alignment may be performed first, followed by a crude approach ofthe cover plate subassembly to the PCB (at which point the conductiveepoxy may or may not be in contact with both the cover plate subassemblyand the PCB), followed by a fine alignment, followed by bringing thecover plate portion and the PCB portion together to a final distancefrom one another. Step 538 utilizes UV light or a thermal bake to curethe conductive epoxy applied in step 530, to complete the assembly ofthe PCB assembly. An optional step 540 applies fill material to spacesbetween the PCB and the cover plate, as described in connection withFIG. 9A.

While the trend in the solid-state lighting industry is to employ largeLED chips, small LED chips often have a much greater extractionefficiency of light because light emitted laterally within the chip hasa greater chance of being emitted from the chip rather than interactingwith the semiconductor material before emission can occur. However,total light output (lumen output) is reduced as the chip dimensions arereduced. Therefore, a large number of small LED chips may be used tocompensate for the lower levels of light generated by each small LEDchip.

FIG. 16A schematically shows a prior art LED lighting product 600.Lighting product 600 includes a mechanical fixture 602 in which aremounted LED light modules 604. Each light module 604, in turn, includesan array of three large LED chips 30(3).

FIG. 16B shows an LED chip-based lighting product 650. Lighting product650 includes a panel 654 within a frame 656. Lighting product 650 maybe, for example, a troffer. Panel 654 may be made of (a) an electricallyinsulating material, or (b) an electrically conductive material coatedwith an insulating material; for example, panel 654 may be a paintedmetal panel. Many small LED chips 30(4) mount on a first surface 653 ofpanel 654. LED chips 30(4) are, for example, stochastically arranged onfirst surface 653 such that no lines, grids or other regular patternsare evident. LED chips 30(4) may range in size from 0.25 mm² to 4 mm². Asecond surface 652 of panel 654 counterfaces first surface 653; that is,surface 652 is on an opposite side of panel 654 from surface 653. Atleast part of second surface 652 is an external surface of lightingproduct 650. In certain embodiments, one or more of LED chips 30(4) areattached by wirebonds to conductors formed on first surface 653 (e.g.,see FIG. 18). In certain embodiments, one or more of LED chips 30(4) maycouple with a cover plate assembly as substantially described above andillustrated in FIGS. 11A-11C. The stochastic arrangement of LED chips,use of small chips, and their use on a panel that forms an externalsurface of lighting product 650 is advantageous from the perspective ofa user for several reasons, including: (1) Human users often findregular patterns of visible objects such as light sources distracting,and such patterns can cast light in stripes or other distracting lightpatterns, whereas stochastic arrangements are less distracting and maycast light more evenly. (2) The smaller LED chips produce less light perchip, such that the individual point sources do not produce discomfortif viewed directly. (3) Smaller LED chips may be more efficient thanlarger LED chips at emitting all of the light produced, so that netenergy efficiency is higher. (4) Heat dissipation from each LED chip30(4) does not pass first to a PCB, then to other structures of lightingproduct 650; instead, the backside of each chip 30(4) forms a directthermal interface, as defined below, through second surface 652 of panel654 and then to ambient air. Lighting product 650 may includetransmissive or translucent screens or diffusers as discussed below;such screens or diffusers are not shown in FIG. 16B for clarity ofillustration.

FIG. 17A is a cross-sectional view of LED chip-based lighting product650. LED chips 30(4) are shown mounted with conductors 658 on firstsurface 653 of panel 654, so that one of the electrical contacts of eachLED chip is through one of the conductors 658. In an alternativeembodiment, LED chips 30(4) mount directly to first surface 653 of panel654 and all electrical connections are made through the top side of eachLED chip. A frame 656 attaches to panel 654 and holds an optionaldiffuser 660. Second surface 652 of panel 654, counterfacing firstsurface 653, is an external surface of lighting product 650 and is inthermal communication with ambient air 670. The term “ambient air”herein denotes air entirely outside a lighting product, and excludes airwithin enclosed cavities of the lighting product. In the embodiment ofFIG. 17A, all of second side 652 forms an external surface of lightingproduct 650; however in other embodiments an external surface may beformed by only a portion of a second side of a panel. A region withinLED chip-based lighting product 650 is denoted as A and is described infurther detail below.

FIG. 17B is a detail view of region A shown in FIG. 17A. Spacingsbetween adjacent LED chips 30(4) are denoted as S. While the value of Svaries among the various adjacent pairs of LED chips 30(4) in lightingproduct 650, an average value S_(avg) of all such adjacent pairs of LEDchips may be determined. Also in FIG. 17B, a standoff distance fromsurfaces of LED chips 30(4) to optional diffuser 660 is denoted as D.When D is greater than or equal to about two times S_(avg), light fromadjacent LED chips 30(4) blends together on diffuser 660 such that anoverall light distribution from lighting product 650 appears to a useras a somewhat homogeneous lighted area, instead of a collection ofindividual points or spots of light. The appearance of a lighted areainstead of points of light may be preferable and less distracting to auser of lighting product 650. A region within region A is denoted as Band is described in further detail below.

FIG. 17C is a detail view of region B shown in FIG. 17B and shows LEDchip 30(4) mounted with conductor 658 formed on first surface 653 ofpanel 654. A backside surface of LED chip 30(4) that faces conductor 658is shown as mounting surface 672. An arrow T illustrates a directthermal interface from LED chip 30(4) that extends perpendicularlythrough mounting surface 672, conductor 658 and panel 654 to ambient air670. The term “direct thermal interface” herein, when used in connectionwith an LED chip, denotes an arrangement of at most one conductor, onepanel and intervening mounting materials (e.g., solder, epoxy oradhesive) that extends perpendicularly from a backside of the LED chipto ambient air. A direct thermal interface thus excludes arrangementsthat require heat transfer in one or more lateral directions (anydirection that is not perpendicular to an LED chip's mounting surface,e.g., surface 672) to reach ambient air, and arrangements that transferheat from an LED to enclosed cavities. Used in connection with apackaged LED, the term “direct thermal interface” denotes a similararrangement of at most one conductor, one panel and intervening mountingmaterials (e.g., solder, epoxy or adhesive) that extends perpendicularlyaway from a light emitting side of the packaged LED to ambient air.

FIG. 18 shows a portion of an embodiment of an LED chip-based lightingproduct 700. Lighting product 700 includes a connection 704 to anexternal power source. Connection 704 may be, for example, a power cordor wiring enclosed within a conduit. The external power source (notshown) connects, through connection 704, to optional power conversionelectronics 701 through an aperture 703 in a light fixture 702. Powerconversion electronics 701 may include power conditioning and controlcomponents, for example (a) to convert AC power to DC power, and (b) toregulate voltage and/or current of the DC power as needed for LEDs(e.g., to convert high input voltage to a maximum voltage usable withLEDs, and/or to vary light output of the LEDs for dimming); steps (a)and (b) may be done in any order or repeatedly. Conductors 706 areformed on an upper panel 710 of light fixture 702 by conventional meanssuch as silk screening, inkjet printing, photolithography,electroplating and/or other methods known in the art. LED chips 30 mountto one or more conductors 706, typically through the use of automatedpick and place equipment.

FIG. 18 shows LED chips 30 connected to power conductor 706(2) viawirebonds 712, but other means of establishing connections from LEDchips 30 to conductor(s) may be used. Also, it is appreciated that ifLED chips 30 are fabricated with two backside electrical connections(e.g., on the opposite side of the chips from their emitting surfaces),that connections may be made to conductors 706 with the electricalconnections facing downwardly (e.g., facing panel 710) and the emittingsurface facing upwardly (e.g., emitting outwards from panel 710). (Notall LEDs 30 or their associated wirebonds 712 are labeled in FIG. 17,for clarity of illustration.) Conductors 706 are shown in FIG. 17 as onelarge ground conductor 706(1) (e.g., a ground connection for the LEDchips) interdigitated with one power conductor 706(2) (e.g., a powerconnection), but it is appreciated that conductors 706 may be routed inany convenient fashion utilizing automatic routing tools as are known inthe art of PCB design. Power conversion electronics 701 supplyelectrical power to conductors 706 via electrical traces 705 that maybe, for example, leads extending from power conversion electronics 701that are soldered to conductors 706. LED chips 30 are mounted ontoconductors 706 using conductive epoxy, solder, or other known materialsand methods for attaching semiconductor chips to substrates.

LED chips 30 may, optionally, be coated with one or more materials thatmay include protective substance(s) for protection from contaminantsand/or phosphors for downconverting wavelength of a portion of the lightemitted by each LED to a longer wavelength (e.g., to create “white”light from LED chips emitting mostly blue light). Alternatively, one ormore cover plates may be applied to individual LED chips, groups of LEDchips or an entire fixture. Although the surface area of light fixture702 is inherently conducive to heat dissipation, extra heat sinks suchas fins (not shown) may be added to light fixture 702 to further promoteheat dissipation.

LED chip-based lighting products may be assembled in several ways at afixture level, that is, in terms of integrating a substrate or panel towhich the LED chips are attached, to support structure of the lightingproduct. FIG. 19 shows one exemplary flowchart of a method 800 forassembling LED chip based lighting products. Method 800 describesassembly of a lighting product exemplified as based on a mechanicalfixture for a troffer type lighting product; however it is appreciatedby those skilled in the art that the techniques described therein may beadapted to assembly of other lighting products. It is also appreciatedthat certain steps of method 800 may be omitted or performed in an orderdifferent than that shown in FIG. 18.

Step 810 patterns conductors on a panel that will eventually form a topinside surface of the lighting product. In this way, method 800 takesadvantage of the fact that the step of patterning the conductors (e.g.,screen printing, or photolithography and etching of conductive layers)may be more easily performed on a flat panel, than on a panel that isnot flat. Step 810 may implement a stochastic layout for a large numberof LEDS on the panel, as discussed previously.

Method 800 alternatively includes step 820 or step 822, which are shownas following step 810, but it is appreciated that step 820 or 822 may beperformed later in method 800, such as after any of steps 830, 840 and850, discussed below. Step 820 bends the panel to form the mechanicalfixture, and step 822 mounts the panel to a frame to form the mechanicalfixture. In one embodiment, step 822 involves mounting (e.g., screwingor bolting) a frame to edges of the panel. Alternatively, step 822 mayplace the panel with LED chips atop another panel and the two panels maybe bonded together. In this case, one of the panels is preferablyductile or compliant such that intimate thermal contact is providedbetween the panels when dust or other contamination blocks directcontact between the two panels at one location. Alternatively, athermally conductive paste or adhesive may be utilized to join the twopanels.

Step 830 mounts LED chips to the conductors. As previously discussed,step 830 may include picking and placing the LED chips from stretchedtape or from die carriers, and may include affixing the LED chips to theconductors utilizing conductive epoxy, solder, or other known methodsfor attaching a chip to a conductor. An optional step 840 forms top sideelectrical connections of the LED chips to the lighting product. Step840 is not needed when step 830 includes establishing all neededelectrical connections of the LED chips through their bottom sides(e.g., when the LED chips have both P and N contacts on their bottomsides). Step 840 may include, for example, forming wirebond connectionsfrom bonding pads on top sides of the LED chips to the conductors formedon the panels in step 810. Step 840 may also be performed in conjunctionwith step 860 discussed below, when protective material in the form of acover panel is affixed over the LEDs.

Step 850 mounts external electrical connections, and non-LED electricalcomponents, to the mechanical fixture. At a minimum, step 850establishes some means for transferring electrical power into thefixture; the electrical power may be controlled external to the fixture,or means for transferring raw power (e.g., 120VAC power) may beestablished, along with power conditioning and control elements toconvert the raw power into an appropriate power source for the LEDs. Forexample, an external power connection in the form of wires in a conduitmay be brought through an aperture in one side of the fixture, the wiresmay connect to a PCB having power conditioning components thereon, andconditioned power for the LEDs may be transferred to the conductors byconnections soldered between the PCB and the conductors.

Step 860 applies one or more materials, such as protective materialsand/or phosphors, to the LED chips and/or to other components (e.g., thecomponents mounted in step 850). In one example of step 860, transparentepoxy is applied over each LED chip so that when the epoxy cures, theLED chip is protected but can emit light through the epoxy. The epoxymay include one or more phosphors for downconverting a portion of thelight from the LEDs to longer wavelengths. Another example of step 860is mounting one or more cover plates or lenses over one or more LEDchips at a time, as described in conjunction with FIGS. 8A through 14C.A single cover plate may be applied over the entire panel. When one ormore cover plates are utilized, electrical connections may be formedsimultaneously and fill materials, optionally including a phosphor, maybe applied, as also described in conjunction with FIGS. 8A through 14C.It is appreciated that application of a phosphor alone may take placefor example between steps 830 and 840; that is, the phosphor may beapplied to mounted LED chips before wirebonding occurs, so as tominimize risk of damage to wirebonds by applying the phosphor later.

An optional step 870 adds diffusers and/or heat sinks to the lightingproduct. Diffusers serve to further spread out light from the individualLED chips to reduce glare, and may be transparent, translucent or gratetype elements. Grate type elements serve to reduce cut-off angle of alight fixture, which is an angle from a user to a light fixture wherethe light emitting elements themselves are no longer visible to theuser. Human users of lighting products often prefer high cut-off angles,that is, the users find it preferable not to be subjected to glare oflight sources more or less in their line of sight, but rather to havesuch sources at a high angle where human eyebrows form a natural glareshield. Grate type elements may be particularly advantageous in LEDchip-based lighting products, by serving the dual functions ofincreasing cut-off angle and as additional heat sinking elements.Another type of diffuser that may be particularly advantageous is adiffuser that forms upwardly pointed shapes (e.g., pointed towards theLED chips), as each such shape will tend to split light incident upon itinto a set of rays at differing angles. In so doing, from a user'sstandpoint, the shapes divide each point source into a distributed setof point sources, thus effectively splitting the light sources intomultiple, fainter light sources that will be less distracting than theoriginal, brighter sources. Transparent and translucent diffusers canalso include phosphors or pigments for adjusting the spectral output ofthe lighting product.

Heat sinks may also be applied in step 870—typically to the top side ofthe panel (opposite the side where the LEDs are mounted)—to improve heattransfer away from the lighting product. The product may also beventilated to encourage convective flow for heat removal. This runscounter to the prevailing practice in design of fluorescent fixtures, inwhich heat is often intentionally concentrated by providing a closedcabinet, since fluorescent ballasts and tubes are often more efficientat high temperatures.

In addition to a more evenly distributed light pattern, stochasticdistribution of LED chips in an LED chip-based lighting product resultsin even and efficient thermal dissipation. Conventional modular arraysmay localize heat buildup to a location where modules attach, creatingareas of localized high temperature that may require large,area-specific heat sinks. By directly integrating unpackaged LED chipsinto a light fixture to form direct thermal interfaces, large heat sinksmay be eliminated. In embodiments of LED chip-based lighting products,the light fixture itself is used as the heat sink due to the directcontact of the LED chips with the large surface area of the fixture.However, should increased heat dissipation be desired, additional heatsinks may be used to increase the surface area of the fixture.

Certain of the principles outlined above are useful with packaged LEDsas well as with LED chips, that is, certain but not all of theadvantages and economies discussed will be obtained. FIG. 20 shows across section of an LED-based lighting product 950 that mounts packagedLEDs 930 directly onto conductors 958 patterned on a panel 954. Likepanel 654 shown in FIGS. 16B and 17A through 17C, a first surface ofpanel 954 counterfaces a second surface of panel 954, and at least apart of the second side forms an exterior surface of lighting product950. One or more of LEDs 930 is mounted to form a direct thermalinterface to ambient air 970 through only conductors 958 and panel 954.LEDs 930 may be low power LEDs and are mounted in a widely distributedstochastic arrangement (e.g., like LED chips 30(4) on panel 654, FIG.16B. A frame 956 attaches to panel 954 and holds an optional diffuser960 at a distance from LEDs 930 at least twice an average spacingbetween adjacent LEDs 930 on panel 954 (the distance and spacing are notlabeled in FIG. 20 but may be determined in the same way as the distanceand spacing illustrated for LED chips in FIG. 17B). By directly mountingpackaged one or more LEDs on a panel of which at least a part forms anexterior surface, lighting product 950 streamlines thermal dissipationand lowers manufacturing cost as compared with an approach that firstmounts packaged LEDs on a PCB or other module, then mounts the module ina lighting product.

Example 1

In one embodiment, a common 4′×2′ troffer fixture provides 8 square feetof surface area that is used as a heat sink. The fixture is constructedof aluminum which results in a 4× increase of thermal conductivitycompared to steel. Electrical connections to the LED chips are patternedonto the light fixture by known screen printing, inkjet technology orother means. Automated pick and place equipment places the LED chips inthe correct positions, and wire bond equipment is used to connect theelectrical traces. Five hundred 50 micron LED chips are stochasticallydistributed as a widely distributed stochastic arrangement over the 8square feet of the aluminum fixture. Each 50 micron LED chip delivers alumen output of 10 lumens. Thus the stochastic array of 500 LED chipsproduces a 5000 lumen output. Each LED chip is driven at around 3.4 V inthe 30 mA to 50 mA range, resulting in a power distribution ofapproximately 6 to 10 W/ft², and the resulting heat is efficientlydissipated through a direct thermal interface from each LED chip toambient air. Light distribution is maximized through the use of highefficiency first surface diffuse reflectors with optical cavitystructures to both minimize glare and optimize light distribution. Atemperature increase (LED to ambient air) generated is between about 20°F. to about 60° F. depending upon the exact configuration of the trofferand the stochastic array. Modification of the troffer fixture mayincludes additional thermal fins or heat sinks, should additionalcooling be desired.

In addition to more even distribution of light, stochastic arrays ofsmall LED chips may result in greater overall energy efficiency. Ohmiclosses within each LED converts part of the power used to drive the LEDinto heat. Equations that help illustrate this include:

V=I·R (“Ohm's law”),  (Eq. 1)

-   -   where V=voltage, R=resistance and I=current, which can be        restated as:

P=I ² ·R  (Eq. 2)

-   -   where P=V·I=power loss through ohmic resistance; and

R(spreading resistance)=I·A·ρ  (Eq. 3)

-   -   where l is the path length, A is cross-sectional area of an LED,        and ρ is the bulk resistivity of the LED material (resistance        per unit volume).

A larger LED chip has a longer path length l, which leads to higherresistance (Eq. 3) and thus higher ohmic power loss for a given currentI (Eq. 2). Thus, if an LED chip size is reduced, power loss is reducedresulting in an improved overall efficiency of the system.

By directly mounting and integrating LED chips into light fixtures,manufacturing costs are greatly reduced by eliminating packaging such asmetal core boards that are most often used to support the packaged LEDs.Not only does eliminating the module level PCBA reduce cost, thermaltransfers between the LED chips and their packages, LED engine PCBAs,and structural elements and/or heat sinks to ambient air are minimizedor eliminated.

Directly mounting the LEDs in the fixture eliminates many costcomponents, flattens the supply chain, and provides a viable path toachieving performance and cost goals. Furthermore, many additionalbenefits may be realized, such as improved thermal dissipation, loweredcomplexity and cost of the thermal dissipation system, lower demand ondrive circuitry, improved drive circuitry reliability and greateroverall system redundancy and reliability.

The changes described above, and others, may be made in thechip-in-fixture methods and systems described herein without departingfrom the scope hereof. For example, although embodiments herein havebeen illustrated with drawings showing certain numbers of LED chips, itshould be clear that any number of LED chips may be incorporated into anLED based lighting product and that such chips may be arranged in twodimensional arrays or in stochastic two dimensional layouts. Theembodiments herein are not limited to specific types of electricalrouting shown in the drawings; LED chips may for example be connected inseries or in parallel, using any number of conductors on panels, PCBs orcover plates, to connect topside or backside contacts. Phosphor layersand types may be single or multiple (for example, to provide multiplefluorescence wavelengths for broad spectrum light) and phosphors may beadmixed with epoxies, conformal gels, index matching gels, fill materialor cover plate materials. It should thus be noted that the mattercontained in the above description or shown in the accompanying drawingsshould be interpreted as illustrative and not in a limiting sense. Thefollowing claims are intended to cover all generic and specific featuresdescribed herein, as well as all statements of the scope of the presentmethod and system, which, as a matter of language, might be said to fallthere between.

What is claimed is:
 1. A method of building a light-emitting diode (LED)chip-based lighting product, comprising: mounting a plurality ofunpackaged LED chips directly on conductors formed on a first surface ofa two-sided panel that includes a second surface counterfacing the firstsurface; integrating the panel with support structure to form thelighting product such that at least part of the second surface forms anexternal surface of the lighting product; and coupling a diffuser withthe support structure, with a distance from the diffuser to the surfaceof the LED chips being at least twice an average spacing betweenadjacent ones of the LED chips.
 2. The method of claim 1, furthercomprising coupling power conversion electronics with the conductors. 3.The method of claim 2, wherein coupling the power conversion electronicsincludes providing means for regulating power to the conductors.
 4. Themethod of claim 1, further comprising applying one or more materials toone or more of the LED chips after the step of directly mounting thechips.
 5. The method of claim 4, wherein applying the one or morematerials comprises applying one or more phosphors to the one or more ofthe LED chips.
 6. The method of claim 4, wherein applying the one ormore materials comprises applying one or more protective materials tothe one or more of the LED chips.
 7. The method of claim 1, furthercomprising forming electrical connections among the conductors and a topside of one or more of the LED chips.
 8. The method of claim 7, whereinforming the electrical connections comprises attaching a cover platehaving at least one conductor patterned thereon, to the one or more LEDchips, such that the one or more LED chips make electrical contact withthe conductor on the cover plate, and light emitted from the one or moreLED chips passes through the cover plate.
 9. The method of claim 8,wherein attaching the cover plate comprises attaching the cover platewith the conductor on the cover plate being transparent, such that lightemitted from the one or more LED chips passes through the conductor onthe cover plate.
 10. The method of claim 1, further comprising mountingone or more reflectors on the panel to reflect light from the LED chips.11. The method of claim 1, wherein integrating the panel with supportstructure comprises forming the panel to create the support structure.12. The method of claim 1, wherein integrating the panel with supportstructure comprises attaching the panel to a frame.
 13. The method ofclaim 1, wherein mounting the plurality of unpackaged LED chipscomprises mounting the LED chips in attachment sites that arestochastically arranged on the first surface such that no lines, gridsor other regular patterns are evident in the attachment sites.
 14. Themethod of claim 1, wherein directly mounting comprises forming a directthermal interface for one or more of the LED chips to ambient air.
 15. Amethod of building a light-emitting diode (LED) based lighting product,comprising: mounting a plurality of LEDs directly on conductors formedon a first surface of a two-sided panel that includes a second surfacecounterfacing the first surface; integrating the panel with supportstructure to form the lighting product such that at least part of thesecond surface forms an external surface of the lighting product; andcoupling a diffuser with the support structure such that a distance fromthe diffuser to the surface of the LEDs is at least twice an averagespacing between adjacent ones of the LEDs.
 16. A method of building alight-emitting diode (LED) chip-based lighting product, comprising:mounting a plurality of unpackaged LED chips directly on one or morefirst conductors formed on a first surface of a two-sided panel thatincludes a second surface counterfacing the first surface, the firstsurface of the panel also having one or more second conductors formedthereon; and mounting a cover plate to the LED chips such that lightemitted from the LED chips passes through the cover plate.
 17. Themethod of claim 16, wherein mounting the cover plate comprises (a)mounting the cover plate having third conductors formed thereon, and (b)forming electrical connections between top sides of one or more of theLED chips and the second conductors, by connecting the top sides of theLED chips with the third conductors and connecting the second conductorswith the third conductors.
 18. The method of claim 16, furthercomprising positioning one or more reflectors between the first surfaceand the cover plate such that a portion of light emitted by the LEDchips is reflected by the one or more reflectors through the coverplate.
 19. The method of claim 18, wherein positioning the one or morereflectors comprises positioning one or more reflectors formed of one ofmetal-coated plastic and micromachined silicon.
 20. A method of buildinga light-emitting diode (LED) chip-based lighting product, comprising:mounting a plurality of unpackaged LED chips directly on conductorsformed on a first surface of a two-sided panel that includes a secondsurface counterfacing the first surface, each of the LED chips beingbetween 0.25 mm² and 4 mm² in area, the unpackaged LED chips dissipatingpower of 6 to 10 Watts per square foot of area of the panel; andintegrating the panel with support structure to form the lightingproduct such that at least part of the second surface forms an externalsurface of the lighting product.